Air separation employing waste nitrogen reheated by incoming air in work expansion



June 3, 1969 a K. c. sM|1H; 3,447,331

AIR SEPARATION EMPLOYING WASTE NITROGEN REHEATED BY INCOMING AIR IN WORK EXPANSION Filed may 9. 1967 CASEOUS ARCO" PURE N2 PURC N2 UNDER PESSURE WASTE N2 mW/Wr- United States Patent O U.S. Cl. 62--13 6 Claims ABSTRACT OF THE DISCLOSURE Air is cooled in reversing exchangers and separated into nitrogen and oxygen product streams in a double rectification column. Waste nitrogen from the low pressure c01- umn is heated in the reversing exchanger and is then Work expanded to provide refrigeration for the process.

The present invention relates to an air separation process.

Many industrial processes, mainly in the metallurgical and chemical synthesis fields, require supplies of the product streams of air separation on the tonnage scale; that is to say upwards of 50 tons per day and exceeding 500 tons per day of gaseous oxygen or nitrogen products. A measure of flexibility is necessary in the operation of such air separation plants to vary both the degree of purity of such product streams and the pressure at which they are delivered. It is also advantageous to produce argon and the other rare gases as required.

Hitherto, such fiexibility has been achieved only by providing both low pressure and high pressure air streams with expansion of the high pressure stream; also compression and expansion of the cold separation products. This has necessitated the provision of a relatively complex system of compressors and expanders, expensive in prime cost and in maintenance and surveillance.

It is an object of this invention to provide an air separation process for producing gaseous separation product streams under pressure which avoids the need both for machinery for air compression to a high pressure and for pumping liquid product streams under pressure.

According to this invention a process for separating air by double rectification in relatively high pressure and relatively low pressure zones to produce .at least one relatively high purity oxygen product stream, and a waste nitrogen product stream, comprises compressing air to relatively high pressure, cooling the compressed air in reversing heat exchangers by, at least, part of said product streams, feeding the cooled compressed air into the high pressure rectification zone to effect therein -a primary separation into at least one relatively pure nitrogen fraction and a relatively pure oxygen fraction and transferring at least a part of said nitrogen fraction and said oxygen fraction to the relatively low pressure rectification zone to effect therein further separation into product streams characterized by the following three features in combination, namely,

(i) That all product streams are withdrawn from the double rectification system at pressures of 1.5 atmospheres or higher;

(ii) That the whole of the cold requirements for the separation process are provided by work-producing expansion of at least one of the said product streams to substantially 1 atma. pressure; and

(iii) That prior to expansion the product stream or streams which are to undergo expnsion are heated to effect thermal balancing of the reversing heat exchangers.

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The air may be separated into product streams including the following:

(l) A relatively pure oxygen product stream withdrawn from the relatively low pressure rectification zone at a pressure of at least 1.5 atmospheres;

(2) A relatively pure nitrogen product stream which is withdrawn from the relatively high pressure rectification zone at the pressure thereof;

(3) A waste nitrogen product stream which is withdrawn from the lower pressure rectification zone at a pressure of at least 1.5 atmospheres and which subsequently undergoes work-producing expansion to substantially atmospheric pressure thereby producing the total refrigeration requirements for the separation process.

If a pure nitrogen stream is not required, the available cold may be utilized to produce a substantial proportion of the oxygen as liquid.

At least part of the waste nitrogen product stream is heated prior to the work-producing expansion step. This heating may be achieved by passing a part of the waste nitrogen through the reversing heat exchanger. Alternatively the reversing heat exchanger may be balanced by so-called, Trumpler pass or by side bleed air, this air being used to preheat the waste nitrogen.

A further relatively .pure nitrogen product stream may be separated in the lower pressure rectification zone and withdrawn therefrom at a pressure of at least 1.5 atmospheres.

Yet another product stream may be withdrawn from the relatively low pressure rectification zone, said further product stream being enriched in argon and being subjected to rectification in an auxiliary rectification zone at a pressure of at least 1.5 atmospheres to produce a substantially pure argon product.

In a modified process, instead of withdrawing a relatively pure nitrogen product stream from the relatively high pressure rectification zone, a substantially pure nitrogen product stream is withdrawn from the relatively low pressure rectification zone of at least 1.5 atmospheres.

The substantially pure nitrogen product stream may be subjected to Work-producing expansion to augment the refrigeration produced by the expansion of waste nitrogen product stream.

A simplified air separation cycle in which air is subjected to two-stage rectification at relatively high and relatively low pressures is shown in the accompanying drawing which provides for the production of four gaseous product streams, namely.

(1) Oxygen of 98.5% purity at relatively low pressure;

(2) Nitrogen substantially pure `at relatively high pressure;

(3) Nitrogen of high purity at relatively low pressure;

(4) Argon at relatively low pressure.

The term high pressure as used herein means a pressure approximating to the supply pressure of the air stream. This pressure will he in the range 7 to 15 atmospheres preferably 7.5 to 12 atmospheres.

The term low pressure as used herein -means a pressure approximating to that of the low pressure rectification zone.

Referring to the drawing, air drawn in through filter unit 1 enters a turbocompressor 2 driven by a prime mover 3 wherein it is compressed to about 7.8 atmospheres. The air from the turbocompressor 2 passes into a direct cooler 4 wherein it is cooled by water to remove heat of compression. After passing through a moisture separator 5, the compressed air is directed through a reversing heat exchanger system 6a and 6b wherein it is cooled to the dew point by heat exchange with separation product streams. The cooled compressed air stream then passes through a gas phase -adsorber system 9 to remove CO2, residual moisture and other impurities.

From the adsorber system 9, the compressed ,air is fed through line 20 at substantially 7.8 atmospheres to the lower, relatively high pressure, rectiiication zone of a conventional double rectication system comprising relatively high pressure column 12, condenser 11 and relatively low pressure column 10. As is well known in the art, in the relatively -high pressure column 12 the air is separated into an oxygen enriched fraction, a nitrogen enriched fraction and a substantially pure nitrogen fraction.

The oxygen enriched fraction is withdrawn through line 21 and after expansion through a valve 22 is fed into the upper, relatively low pressure, column which operates at approximately 1.5 atmospheres.

The nitrogen enriched fraction is withdrawn from column 12 through line 23 and after expansion to approximately 1.5 atmospheres in valve 24 is fed to the upper part of column 10 to serve as reux therein.

The substantially pure nitrogen fraction is withdrawn from column 12 through line 25 and after expansion through valve 26 is fed to the uppermost extremity of column 10 to produce a high purity nitrogen stream.

Oxygen of 98.5% purity accumulating at the lower end of column 10 is withdrawn through line 27 and after passing through the heat exchanger system 6b, 6a, where it transfers its cold to the incoming air stream, emerges as the oxygen product stream at approximately 1.5 atmospheres and ambient temperature.

Substantially pure nitrogen at upper column pressure is withdrawn through line 28 and after passing through heat exchange system 6b, 6a, emerges as a pure nitrogen product at ambient temperature and at a pressure of approximately 1.5 atmospheres.

A waste nitrogen product stream containing about 2.7% oxygen is drawn off through line 29, is passed through tur-bine exhaust cooler 8 and is then split into two streams, one flowing through line 30, the other through heat exchanger unit 6b where it is warmed by the incoming compressed air stream and is then admixed with the cold stream flowing through line 30, the mixed streams then being fed to expansion turbine 7. The exhaust from the turbine after cooling the efuent waste nitrogen stream in exchanger 8 passes through the reversing heat exchanger system to cool the incoming compressed air stream. It is an essential feature of this invention that the cold produced by expansion of the waste nitrogen stream provides substantially all the cold requirements for the separation process. It will be understood that as in all air separation processes involving compression and expansion, la. small amount of cold is produced by the isothermal Joule-Thomson elect during compression of the ingoing gas.

It will ibe understood that the passages in the heat exchanger system through which the incoming air stream on the one hand and the expanded waste nitrogen stream on the other hand pass are periodically switched over in order that condensible impurities which are precipitated from the air stream may be revaporised in alternating cycles by the waste nitrogen stream.

If it is desired to separate a gaseous argon fraction a side-column 13 is provided. A fraction in column 10 which is enriched in argon is supplied to column 13 through line 31. Fractional condensation is effected in column 13 against a part of the oxygenenriched fraction taken from line 21 and expanded through valve 32. The oxygen-enriched fraction vaporised in column 13 is returned to line 21 on the downstream side of expansion valve 22 Whilst the condensed part of the oxygen-enriched fraction which forms in column 13 is fed through line 33 to column 10 at a point below the takeot point of line 31. Gaseous argon at the pressure of column 10 is taken off from column 13 through line 34 and passed through a separate passage in the heat exchange system 6b, 6a.

A part of the separated oxygen or nitrogen or both may be produced in liquid form from the double column system in known manner.

I claim:

1. Process for separating air by double rectiiication in relatively high pressure and relatively low pressure zones to produce at least one relatively high purity oxygen product stream, and a waste nitrogen product stream, comprising compressing air to relatively high pressure, cooling the compressed air in reversing heat exchangers by, at least, part of said high purity oxygen and waste nitrogen product streams, feeding the cooled compressed air into the high pressure rectiiication zone to effect therein a primary separation into at least one relatively pure nitrogen fraction and a relatively pure oxygen fraction and transferring at least a part of said relatively pure nitrogen fraction and said relatively pure oxygen fraction to the relatively low pressure rectification zone to eiect therein further separation into said relatively high purity oxygen and waste nitrogen product streams, said process being characterised by,

(i) withdrawing said relatively high purity oxygen and waste nitrogen product streams from the low pressure rectification zone at pressures of at least 1.5 atmospheres;

(ii) providing substantially the whole of the cold requirements for the separation process by work-producing expansion of at least one of the said product streams from the low pressure rectication zone to substantially l atmosphere pressure; and

(iii) effecting thermal balancing of the reversing heat exchangers by heating a portion of said one of said product streams in the reversing exchanger prior to expansion said one of said product streams which are to undergo said work-producing expansion.

2. Process according to claim 1 which comprises withdrawing the waste nitrogen product stream from the lower pressure rectification zone at a pressure of at least 1.5 atmospheres and subsequently allowing the waste nitrogen to undergo said work-producing expansion to substantially atmospheric pressure thereby producing the total refrigeration requirements for the separation process.

3. Process according to claim 1 which comprises utilising the available cold to produce a substantial proportion of the oxygen as liquid.

4. Process according to claim 1 which also comprises producing at least one relatively high purity nitrogen product stream.

5. Process according to claim 4 which comprises withdrawing the relatively high purity nitrogen product stream from the relatively high pressure rectiication zone at the pressure thereof.

6. Process Iaccording to claim 4 which comprises separating a further relatively pure nitrogen product stream in the lower pressure rectification zone and withdrawing said stream therefrom at a pressure of at least 1.5 atmospheres.

References Cited UNITED STATES PATENTS 2,502,250 3/ 1950 Dennis 62-14 2,918,802 12/1959 Grunberg 62-38 XR 3,070,966 1/1963 Ruhemann et al. `62-39 XR 3,216,206 1l/l965 Kessler 62-13 3,217,502 11/1965 Keith 62-39 XR 3,327,488 6/ 1967 Pervier 62-39 XR 3,340,697 9/ 1967 Cimler et al. 62--13 3,375,673 4/1968 Cimler et al. 62-13 3,261,168 7/ 1966 Ruhemann et al. 62-38 XR NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

U.S. Cl. XR. 62-22, 39 

